A magnetoresistive head is a sensing or reading head which utilizes magnetoresistive elements to sense or read magnetic information inherent in a magnetic medium. A magnetoresistive element is one whose electrical resistance varies with varying incident magnetic fields.
Many prior art magnetic heads, for a great many applications including coin sensing, determining the contents of a coin tube, currency and bill validation, and determination of the denomination of paper currency or other forms of scrip, are of the inductive type which require a rapidly changing magnetic field in order for magnetic information to be detected and recorded. Magnetoresistive heads, on the other hand, are capable of detecting and reading information inherent in a slowly moving magnetic field. Prior art magnetoresistive heads are typically difficult to fabricate and consequently relatively expensive, as further discussed below.
The orientation of a sensing element of a magnetoresistive head may be either vertical or horizontal in relation to the magnetic medium which is being read. In the vertical orientation, described in R. S. Indeck, J. H. Judy, and S. Iwaski, "A Magnetoresistive Gradiometer," IEEE Trans. Magn. 24, 2617 (1988), the magnetoresistive head is situated on the magnetic medium in a vertical or standing up position which places the bottom edge of the head either in contact with or closely adjacent to or near, the magnetic medium which it is desired to read from.
A number of dual-element vertical magnetoresistive sensors have been described in the prior art literature for use with high performance reading. In general, these vertical structures have been comprised of two parallel magnetoresistive elements or sensors placed in close proximity to each other and standing perpendicular to the plane of the magnetic medium to be read. The advantages claimed for such structures include good common noise rejection, a broader range of linear performance (and consequently, a reduction of second-harmonic distortion), and a larger signal per unit width than with single sensor heads.
Vertical magnetoresistive heads are fabricated on a wafer, and then a strip containing a number of these heads arranged in a row is cut from the wafer. This cutting or separating process creates burrs or rough edges on the separated edges. The edge of this strip which contacts the magnetic medium must then be polished. This polishing step is critical because it determines sensor height. As tolerance is typically very tight, the process is an expensive one. The resulting polished strip is then diced to separate the individual heads.
Fabrication of horizontal magnetoresistive heads according to the present invention is much simpler than the above described vertical head fabrication method. In the present invention, once the wafer containing the head has been diced, it is the faces of the diced head elements, and not their side edges, that are to be used to either make contact with, or be placed closely adjacent to, the magnetic medium to be read. Hence, the present invention dispenses with the costly and time consuming polishing process.
While horizontal magnetoresistive heads are described in the prior art, such as by D. W. Chapman, D. E. Heim and M. L. William, "A New, Horizontal MR Head Structure," IEEE Trans. Magn. 25, 3689(1989) and D. W. Chapman, "A new approach to making thin film head-slider devices," IEEE Trans., Magn. 25,3686 (1989), these horizontal magnetoresistive heads employed a shield and the fabrication process employed to make them is premised around the inclusion of this shield. The present invention dispenses with such a shield and provides a horizontal magnetoresistive reading head which is unshielded. As a result, a simpler and less expensive fabrication process is employed to make unshielded horizontal heads according to the present invention. These heads have sufficient resolution to be highly desirable for use in a host of applications not previously recognized as appropriate for more expensive heads, and as a replacement for more expensive heads presently utilized in certain applications.